Our invention concerns a fluid circuit for driving a bidirectional hydraulic motor used, for example, for propelling an earthmover or for actuating a revolving unit thereon. Our invention is directed more specifically to improved means in such a hydraulic circuit for the reduction of shocks arising as when the motor is set into or out of rotation.
The hydraulic motor drive circuit of the class under consideration has generally been equipped with a brake valve arrangement. It comprises a counterbalance valve interposed between the motor and a directional control valve operated manually for selectively placing the pair of inlet/outlet ports of the motor in communication with a pump and a fluid drain, and two safety valves connected in opposite directions between the pair of main fluid lines extending between motor and counterbalance valve. A problem with this type of motor drive circuit has been the production of shocks when the manual control valve is manipulated to set the motor into or out of operation. Shocks have also tended to occur at the end of the acceleration, or at the start of the deceleration, of the motor. These shocks are due to an abrupt pressure rise, caused by the inertial force of the load on the motor, taking place in the driving fluid line at the time of a start up and in the braking fluid line at the end of acceleration, at the start of deceleration, and at the time of a stop.
A solution to the above problem has been suggested by Japanese Utility Model Application No. 57-29614 and corresponding U.S. patent application Ser. No. 471,341, the latter having been filed by Sato et al. on Mar. 2, 1983 and assigned to the assignee of the instant application. That prior application proposes the use of two dual pressure relief valves connected in opposite directions between the pair of main lines extending between manual control valve and motor. Each dual pressure relief valve is closed upon pilot actuation by a timing valve responsive to a pressure differential between the main lines, so that the pressure rise in either of the main lines takes place in two discrete steps. The stepwise pressure rise can appreciably alleviate the resulting shock exerted on the vehicle.
We have found the above solution unsatisfactory, however. The timing valve must pilot actuate either of the relief valves with some delay for temporarily holding each pressure rise at the first or lower one of the two steps. The noted prior application employed to that end a fixed orifice between each of the opposite pilot ports of the timing valve and the associated one of the main fluid lines. We object to the use of the fixed orifices because the length of time during which the fluid pressure is kept at the first of the two successive pressure rises is predetermined by the orifice diameter. Should the fixed orifice diameter be too small, the circuit would become poor in response, demanding a prolonged period of time for braking or accelerating. It has also been found that when the vehicle is steered, the revolving unit thereof would undergo some angular displacement. Should the orifice diameter be too large, on the other hand, then the desired shock reducing effect would lessen.
The above inconveniences arising from the use of the fixed orifices become all the more pronounced in the case of an excavator, which finds a great variety of applications demanding its operation in an as much diversity of ways. Any fixed orifice diameter may suit one mode of operation but will run counter to another. For instance, if the vehicle is put to a duty that requires much steering, the orifice diameter may be so determined as to minimize the undesired angular displacement of the revolving unit at the sacrifice of shock reduction. In another duty, however, the orifice diameter setting may rather be such that shocks are reduced to a minimum even if there is some displacement of the revolving unit.